Digital index cards can be kept closer (on your phone, in your pocket, always), and arguably lower friction too, no? And plain text is pretty flexible too. IDK, I just thought the concept was strange - but if it works for you, that's great!

... and substantially future-proofed.

This is only slightly more future-proof than keeping text files on a computer. So slight, I'd argue the difference doesn't even matter. I don't see the advantages to this behavior over storing them digitally.